The present invention relates to a pneumatic radial tire and, more particularly, to a pneumatic radial tire which have an improved tire durability and an improved heat build-up suppressing property in its shoulder portions.
Tires typically have a-construction such that a carcass, edge portions of a belt, an under-belt pad and the like are intricately combined with each other in its shoulder portion: The under-belt pad provided continuously along the circumferential shoulder portion of the tire between a tread rubber portion and the carcass and extending outwardly of the belt edge portions along the width of the tire is a thick rubber layer, which is a structural characteristic for alleviating a shear stress possibly generated between the belt edge portions and the carcass. Further, since the under-belt pad is repeatedly subjected to loads during running, heat is liable to build up in the under-belt pad, thereby causing internal rubber destruction in the under-belt pad and adhesion failures between the rubber components and between a rubber portion and-cords (steel cords) in the carcass. This causes separation of the belt edge portions and ply separation in the carcass resulting in breakdown of the tire. One conventional approach to this problem is that the under-belt pad is formed of a rubber compound which contains a reduced amount of carbon black for suppression of the heat build-up.
However, the rubber compound for the under-belt pad is softened by the reduction of the carbon black content therein. This also results in the adhesion failure and the internal rubber destruction in the under-belt pad due to the heat build-up, thereby causing the ply separation and the belt separation in the tire during running. Therefore, this approach is not satisfactory in terms of the durability of the tire. The deterioration of the durability of the tire which results from the heat build-up attributable to the structural characteristic of the under-belt pad is a more critical problem, since the recent performance improvement of automobiles requires that tires have a higher durability under higher speed running and heavy loads.
In accordance with one aspect of the present invention, there is provided a pneumatic radial tire comprising an under-belt pad interposed circumferentially between edge portions of said belt and said carcass in a shoulder portion thereof, the under-belt pad being of a rubber compound containing resorcin or a resorcin derivative and hexamethylene tetramine or a malamine derivative as a methylene donor, and having a dynamic viscoelasticity Exe2x80x2 of 5 MPa to 10 MPa, a loss tangent tan xcex4 of 0.10 to 0.18 and a 300% modulus of 11 MPa to 15 MPa.
Thus, the strength of the shoulder portion including the carcass, the belt edge portions, the under-belt pad interposed therebetween and the like, particularly, the rubber-rubber adhesion strength between the under-belt pad and the carcass and the rubber-cord adhesion strength in the carcass can be enhanced. In addition, the under-belt pad has a low heat build-up property and an appropriate softness. As a result, even with the under-belt pad which has a large rubber thickness and is to be repeatedly subjected to loads during running, the shoulder portion of the tire exhibits a satisfactory durability.
This makes it possible to solve the problems associated with the characteristic structure of the conventional under-belt pad (i.e., the separation between steel ply coads and rubber in the carcass and the separation of the belt edge portions which result from the rubber-rubber adhesion failure, the rubber-cord adhesion failure and the internal rubber destruction in the under-belt pad due to internal heat build-up which may occur due to the large thickness of the rubber under-belt pad and the loads repeatedly applied thereto during running).
A rubber compound having a dynamic viscoelasticity Exe2x80x2 of less than 5 MPa is too soft to be used as a material for the under-belt pad of the pneumatic tire. Even though a satisfactory rubber-rubber adhesion strength and rubber-cord adhesion strength in the tire can initially be ensured with the use of such a rubber compound by the addition of resorcin or the resorcin derivative and hexamethylene tetramine or the melamine derivative as the methylene donor to the rubber compound, the adhesion failure and the internal rubber destruction are liable to occur due to heat build-up resulting from loads applied to the under-belt pad during running, thereby causing the ply separation and the belt edge separation in the tire. Therefore, the rubber compound having a dynamic viscoelasticity Exe2x80x2 of less than 5 MPa is not preferred in terms of the tire durability. A rubber compound having a dynamic viscoelasticity Exe2x80x2 of greater than 10 MPa lacks in softness when used as the material for the under-belt pad of the pneumatic tire. Accordingly, internal heat build-up is liable to occur in the under-belt pad during running even though a satisfactory rubber-rubber adhesion strength and rubber-cord adhesion strength in the tire can initially be ensured with the use of such a rubber compound by the addition of resorcin or the resorcin derivative and hexamethylene tetramine or the melamine derivative as the methylene donor to the rubber compound. Therefore, the rubber compound is not preferred in terms of the tire durability.
A rubber compound having a loss tangent tan xcex4 of greater than 0.18 is not preferable as the material for the under-belt pad of the pneumatic tire in terms of the tire durability. Even though a satisfactory rubber-cord adhesion strength and rubber-rubber adhesion strength in the tire can initially be ensured with the use of such a rubber compound by the addition of resorcin or the resorcin derivative and hexamethylene tetramine or the melamine derivative as the methylene donor to the rubber compound, internal heat build-up is liable to occur in the under-belt pad during running, causing the internal rubber destruction, the rubber-cord adhesion failure, and the rubber-rubber adhesion failure. This results in the ply separation and the belt edge separation in the tire, so that the breakage of the tire is liable to occur. The use of a rubber compound having a loss tangent tan xcex4 of less than 0.10 as the material for the under-belt pad of the pneumatic tire suppresses the heat build-up in the under-belt pad, but reduces the reinforcement effect to reduce the fatigue resistance.
A rubber compound having a 300% modulus of less than 11 MPa is too soft to be used as the material for the under-belt pad of the pneumatic tire. Even though a satisfactory rubber-rubber adhesion strength and rubber-cord adhesion strength in the tire can initially be ensured with the use of such a rubber compound by the addition of resorcin or the resorcin derivative and hexamethylene tetramine or the melamine derivative as the methylene donor to the rubber compound, the internal rubber destruction occurs in the under-belt pad due to loads applied to the under-belt pad during running. Therefore, the durability of the tire is deteriorated. A rubber compound having a 300% modulus of greater than 15 MPa lacks in softness when used as the material for the under-belt pad which has a large rubber thickness and is to be repeatedly subjected to loads during running. Even though a satisfactory rubber-rubber adhesion strength and rubber-cord adhesion strength can initially be ensured with the use of such a rubber compound by the addition of resorcin or the resorcin derivative and hexamethylene tetramine or the melamine derivative as the methylene donor to the rubber compound, the internal rubber destruction is liable to occur in the under-belt pad thereby to deteriorate the tire durability.
As previously described, the under-belt pad has a structural characteristic of a large rubber thickness. In view of this, there is provided, in accordance with another aspect of the present invention, a pneumatic radial tire comprising an intermediate rubber layer interposed between an under-belt pad and a carcass, the intermediate rubber layer being of a rubber compound containing resorcin or a resorcin derivative and hexamethylene tetramine or a melamine derivative as a methylene donor and having a dynamic viscoelasticity Exe2x80x2 of 5 MPa to 10 MPa, a loss tangent tan xcex4 of 0.10 to 0.18 and a 300% modulus of 11 MPa to 15 MPa.
In the case of the pneumatic tire having the intermediate rubber layer interposed between the under-belt pad and the carcass, the strength of the shoulder portion of the tire including the carcass, edge portions of a belt, the under-belt pad interposed therebetween and the like, particularly, the rubber-rubber adhesion strength between the under-belt pad and the carcass and the rubber-cord adhesion strength in the carcass can be enhanced. In addition, the intermediate rubber layer efficiently suppresses the heat build-up in the under-belt pad and allows the under-belt pad to have an appropriate softness. As a result, even with the under-belt pad which has a large rubber thickness and is to be repeatedly subjected to loads during running, the shoulder portion of the tire exhibits satisfactory durability.
Like the rubber compound for the under-belt pad, the rubber compound for the intermediate rubber layer is required to have a dynamic viscoelasticity Exe2x80x2 within a range between 5 and 10 MPa, a loss tangent tan xcex4 within a range between 0.10 and 0.18 and a 300% modulus within a range between 11 and 15 MPa. A rubber compound which does not satisfy the aforesaid property requirements is not preferable as the material for the intermediate rubber layer for the same reasons as described above, even though a satisfactory rubber-rubber adhesion strength and rubber-cord adhesion strength in the tire can initially be ensured with the use of such a rubber compound by the addition of resorcin or the resorcin derivative and hexamethylene tetramine or the melamine derivative as the methylene donor to the rubber compound.
It is, therefore, an object of the present invention to provide a pneumatic radial tire which is prevented from durability deterioration which may occur due to the heat build-up attributable to the structural characteristic of the under-belt pad in the shoulder portion thereof.